The copper catalyst required in the synthesis of methylchlorosilane from methyl chloride and silicon is used in the form of metallic copper such as cement copper, electrolytic copper or oxidic copper prepared from cement copper by pyrometallurgical processes, or copper compounds such as copper oxides, copper hydroxides or other copper salts, or as an alloying constituent of silicon. To improve their activity, stability and reactivity, the copper catalysts contain metals as activators, such as aluminum or zinc, or as promoters, such as tin, antimony or arsenic.
When the direct synthesis is carried out in fluid-bed reactors, fines of silicon and copper catalyst, contaminated with carbon particles and various metal compounds from minor constituents of the technical-grade silicon raw material, are discharged together with the reaction product, the crude silane mixture and unconverted methyl chloride, and are separated off in downstream separating units, e.g. cyclones. A reactor residue consisting of silicon, catalyst and metal halides from minor constituents of the technicalgrade silicon raw materials, and carbon particles, is also obtained and is discharged continuously or discontinuously from the fluid-bed reactors. Drying and filtration processes also result in the production of contaminated dusts of very fine particle size.
The only currently known processes for the recovery of copper from process residues are those where contaminated copper salts are obtained which can only be recycled into the catalyst preparation after expensive purification.
U.S. Pat. No. 4,758,352 (K. Feldner el al., published on Jul. 19, 1988 in the name of Bayer A. G., Leverkusen) describes a process for the treatment of high-boiling residues from the synthesis of methylchlorosilane where the residues are hydrolyzed with water or dilute hydrochloric acid and oxidized with oxygen. The CuCl.sub.2 -containing solution which is separated off is treated with NaOH and SO.sub.2 to precipitate copper(I) oxide.
This copper(I) oxide has to be worked up in known manner, for example by dissolution in dilute sulfuric acid with a supply of air followed by cementation with scrap iron. A further purification of the resulting copper by melt refining and then by an electrolytic process is usually required if fresh catalyst for the synthesis of methylchlorosilane is to be prepared therefrom.
Copper is ductile and cannot be mechanically reduced to a powder with a suitable particle size and surface. If a copper powder is to be prepared which will be processed further to a pulverulent catalyst for the synthesis of methylchlorosilane, for example by oxidation or doping of the copper surface, it has to be prepared by special processes, such as atomization, from the melt.
Therefore, it is an object of the present invention to provide a simple process for the preparation of copper powder which can be utilized as a catalyst in the synthesis of methylchlorosilane. A further object of the present invention is to provide a process for the preparation of copper powder from the copper salt solutions obtained in the working-up of process residues from the synthesis of methylchlorosilane.